(a) Open Hole Non-Rotating Drill Pipe Protector: Recently new drilling and fracturing technology has allowed unconventional development for gas and oil production. Examples of major field developments include the Baaken play in North Dakota, the Marcellus play of Pennsylvania, and the Haynesville play of east Texas and Louisiana. These huge development opportunities have spawned the need for new technologies to develop these resources in these types of wells.
One characteristic of these formations and other formations, especially on land, is that the pay zones may be relatively shallow (5000-12000 feet) and may be relatively thin in their thickness (10-200 feet). These thin formations frequently are exploited by the use of horizontal well profiles, after reaching pay zone depth. When the formations are relatively firm, the hole is frequently not completely cased. Thus, a casing shoe will be placed near the build section (region where the orientation of the wellbore changes from vertical to horizontal). Entrance into and out of the casing with drill pipe or casing is subject to problems of high torque, drag, and buckling.
Another similar problem with respect to drilling into horizontals occurs in multilateral wells. In these wells, multiple sidetrack wells are drilled from a primary wellbore. Again, either drill pipe is run through the sidetrack; or in some cases, slotted liners are installed with the frequent problems of high torque, drag, or buckling.
Another recent development in drilling technology is the use of a single drilling pad to drill multiple directional wells to produce from a reservoir with a minimum of cost and environmental impact. These wells generally have shallow surface casing setting depths. Being directional in nature, they can generate high drilling torque, requiring both larger and more expensive equipment or shallower wells that may result in incomplete access to the reservoir.
An essential part of the drilling and completion of these wells is the drilling with drill pipe, and subsequently, running casing into the hole and cementing the casing into place. A variation of this, that may be used in shallower wells and low angle deviated wells, is to drill with casing and then retract the drilling assembly and cement the casing in place.
For each method, a common problem is that the torque in the drill string may become so excessive that required torque is greater than the top drive (or rotary equipment) and may exceed the capabilities of the equipment. Also, the process of sliding the drilling string downhole while drilling, with or without a motor, may be significant because of the high friction between (1) drill pipe and casing, or (2) drill pipe and open hole formation, or (3) casing and formation, or (4) casing within casing.
(b) Casing Centralizer: Casing centralization is of importance to oil and gas wells because proper centralization of the casing within the hole leads to improved cementing of the casing, and hence, pressure integrity and safety. Centralizers are also important to allow use of slotted liners to avoid slot plugging, reduce drag during installation, and limit differential sticking of the casing to the formation during installation.
Historically, many different attempts were made to satisfy the multiple requirements for proper casing centralization; but these have failed because only one or two of the performance requirements were satisfied in previous designs. These requirements include the need to keep the casing in the center of the hole, allowing the cement to be evenly distributed around the casing. This centralization is difficult because of wellbore configuration and common drilling problems. For example, in non-vertical wells, such as extended reach wells or horizontal wells, the casing's weight forces the casing to the low side of the hole; without centralization, the casing will sit on the bottom side of the hole and prevent proper cementation. Further, certain drilling curvatures occur in the wellbore trajectory caused by variations in rock hardness and orientation; these are commonly called “dog-legs,” and can result in the casing contacting the hole wall in a non-concentric manner.
Also part of casing centralization is efficient passage of the cement past the centralizer towards the surface. If the centralizer fills a significant portion of the annulus between the casing and the wellbore, the result is restriction of the cement flow, thus requiring greater pumping, but more often incomplete cement coverage.
Another common problem occurs when running a smaller casing liner through a casing exit without a whipstock in place. For these applications, failure of the centralizers run on liners through casing exits can result in expensive time lost due to fishing (retrieving parts) and milling of pieces of centralizers in order to obtain proper well function. This significant problem is associated with the transition across the sharp edge of the casing and into open hole.
Another problem with the use of casing centralizers occurs when utilizing casing for drilling operations. This technique utilizes the casing and especially top drive and bottom hole assemblies (BHAs) to drill with the casing, then retrieve the BHA, and cement the casing. Drilling with casing can produce a significant time and cost savings. However, a common problem is that the casing centralizers contact the hole wall and casing, resulting in substantially increased torque, sometimes at or near the limitations of the surface equipment or casing.
(c) Prior Art Non-Rotating Drill Pipe Protectors: Non-Rotating Drill Pipe Protectors (NRDPPs) have been used to reduce torque between drill pipe and casing. (See U.S. Pat. Nos. 5,692,563; 5,803,193; 6,250,405; 6,378,633; and 7,055,631, assigned to Western Well Tool, Inc.) These patents describe particular designs of drill pipe protector sleeves and related assemblies having features that reduce torque, reduce sliding friction, and assist in increasing drill string buckling loads when strategically placed on the drill pipe.
However, these designs have typically been limited to cased hole applications, not open hole applications. A problem may occur with the prior art designs in transitioning from casing to open hole. In some applications, the end of the casing may have washouts that result in a large diametrical difference of the hole to the casing, producing a hazard that can catch the non-rotating drill pipe protector. This can damage one or more NRDPP assemblies, and could result in lost rig time. Also, at casing transitions, the end of the casing can have a sharp edge resulting from the milling process; here again a hazard that can result in snagging the NRDPP at the transition and damaging the sleeve and the NRDPP assembly, possibly resulting in lost rig time and associated expenses. Further, when in open hole the abrasive nature of the formation on NRDPPs of traditional materials can result in excessive wear. Also, many materials used in NRDPPs do little to reduce drag between the drill pipe and the casing; it is advantageous to have designs that reduce drag.
(d) Prior Art Casing Centralizers: Casing centralizers have been used in the past, but with limited success. These include the centralizers disclosed in U.S. Pat. Nos. 5,908,072 to Hawkins, 6,435,275 to Kirk et al., 6,666,267 to Charlton, and U.S. application publication US 2009/0242193 to Thornton. Each of these centralizers has significant deficiencies.
Specifically, Hawkins '072 teaches a tubular centralizer of unitary construction with radially projecting blades. The centralizer contains a cylindrical bore having a bearing surface that makes a close fit around the casing. The centralizer can be bonded to the casing. The contact bearing surface described in Hawkins can have coefficients of friction of 0.30, with its close fit around the casing, thus substantially increasing torque when rotating and running casing into a well.
Kirk et al. '275 teaches a centralizer that has a clearance fit around the casing; but clearance fits result in contact bearing surfaces which produce coefficients of friction of 0.3 for typical plastics, resulting in significantly greater torque at the surface.
Charlton '267 teaches a tubular centralizer sleeve of unitary construction with a clearance fit and ID grooves that taper in depth longitudinally, also non-optimum, because it does not produce or allow a low friction bearing surface that reduces torque at the surface.
Thornton '193 teaches a centralizer also having a clearance fit around the casing, to produce a contact bearing surface that functions as a thrust bearing or a journal bearing during use. The centralizer also contains a polymeric outer sleeve, with an inner liner or tubular end sections of a more rigid material, along with a coating of tungsten disulphide to reduce friction. The performance attributed to the centralizer is not supported by measurements based on use simulating actual downhole environments.
In summary, the current art for casing centralizers used for drilling, or for simply running casing, do not entirely address the combined issues of high torque, high sliding friction, resistance to damage when running over obstacles, and maximizing fluid flow past the centralizer.